Radar level gauge systems are in wide use for measuring process variables of products, such as filling level, temperature, pressure etc. Radar level gauging is generally performed either by means of non-contact measurement, whereby electromagnetic signals are radiated towards a product in a tank, or by means of contact measurement, often referred to as guided wave radar (GWR), whereby electromagnetic signals are guided towards and into the product by a probe acting as a waveguide. The probe is generally arranged vertically from the top of the tank towards the bottom of the tank. The electromagnetic signals are reflected at the surface of the product, and the reflected signals are received by a receiver or transceiver comprised in the radar level gauge system. Based on the transmitted and reflected signals, the distance to the surface of the product can be determined. From this distance, the “ullage”, the filling level of the product in the tank can be determined.
In case of GWR systems, forces, mainly due to friction between the probe and the product in the tank, which act on the probe and on the mechanical connection between the probe and the tank, most commonly the tank ceiling, may be very high. For example, in the case of solids, such as powders or granules, the probe may be subjected to a pulling force well in excess of 40 kN.
As a consequence, the mechanical connection between the probe and the tank should be designed to be able to withstand such high forces. Furthermore, an electrical connection between transceiver circuitry of the radar level gauge, which is typically arranged outside the tank, and the probe should be designed with signal propagation performance in mind, such as signal attenuation and/or impedance matching.
The design of a probe coupling device, which provides for electrical coupling between the transceiver circuitry and the probe is essential in achieving the above-mentioned signal propagation performance.
In general, a rather elaborate design of the probe coupling device is needed in order to simultaneously fulfill these mechanical and electrical requirements.
Additionally, the probe may unintentionally act as an antenna, picking up signals which may interfere with measurement circuitry connected to the probe if not properly taken care of.
In an attempt to address the above issues, US 2004/0046573 discloses a guided wave radar (GWR) level gauging system in which the probe is in metallic connection with a wall of the tank, such that the tensile forces on the waveguide are absorbed by metallic parts, and interfering signals are dissipated by the conductive bulk of the tank walls.